Well, to be brief, a lot of the hate stems from the fact that the US intervened and fought for south Korea's democratic party when the communist party of north Korea tried to take over the Korean back in the 1950's. Since then, the government has cut off ties to the rest of the world (mostly) and has tried to make a perfect society under communism. Thought, I don't seem to understand how refusing international aid during one of the most devastating famine to ever hit the modern world or locking up entire families for one persons crime is how a good society would work. Also, North Korea wants to "reunite" with the South one day and create one, communist Korea. Finally, it helps if you hall a lot of propaganda to back up all that crazy.
Is it's messed up, yes it is and there is much more to it. I suggest that you should at least try to watch the video's that Vice did when they where inside Korea, I found them to be quite good (thought now a bit out dated).
Vice-Inside Korea part 1 out of 3.